The application of active noise cancellation technology to eliminate various noise signals is generally known within the electronics art. Active noise cancellation is a method of reducing noise by locating, next to a noise source, a second noise source that is equal in amplitude, but 180 degrees out of phase with the noise source. The second source is typically called the "anti-noise" source.
Active Noise Cancellation (ANC) exhaust silencing systems typically use one or more speakers, an acoustic enclosure, a microphone, a Digital Signal Processor (DSP) controller, amplifiers to drive the speakers, a power supply, and a synchronous signal. The microphone is mounted near the noise and anti-noise sources and measures the acoustic levels. The synchronous signal is usually derived from an existing speed sensor mounted on the moving machinery or from a sensor which is installed on the machinery. The controller reads the microphone and synchronous sensor inputs and calculates the anti-noise required to cancel the noise. The controller sends the signal to the amplifier which amplifies the noise to the appropriate level and broadcasts the noise through the speakers mounted in the acoustic enclosure. The enclosure has an opening called a port through which the noise is radiated into the atmosphere. This process is repeated continuously by the controller for optimum noise cancellation.
The acoustic enclosure can be designed to produce a large amount of sound over a narrow frequency rage. This type of design is commonly called a fourth order band pass enclosure. The primary enclosure parameters which determine the operational frequency range and the amount of sound produced are the speaker, back volume, front volume, and the port area and length. The back volume is the air enclosed behind the speaker. The front volume is the air in front of the speaker and in contact with the port. The port is the opening that separates the front volume of air from the outside environment. By varying the values and ratios of these parameters, the sound level and bandwidth of the enclosure can be altered to meet the output requirements dictated by the noise source.
The scope of this disclosure primarily covers the application of ANC exhaust silencing systems to the exhausts of large stationary engines and positive displacement blowers. However, these inventions can be used for other exhaust applications such as vehicular, heavy duty, farm equipment, and marine exhausts.
In the past, there was a tendency to have a different enclosure design for each different exhaust pipe size. The exhaust pipe sizes generally range from 3 inches in diameter to 24 inches in diameter. The smaller engines with small exhausts pipes were generally less noisy and only needed one or two speakers and small enclosures to create the appropriate "anti-noise." The larger engines with large exhausts pipes were louder and required more speakers and larger enclosures to create sufficient "anti-noise." The exhaust noise of the different engines and blowers is controlled by several factors including the number of cylinders in an engine or the number of lobes in a blower, the typical load under which the engine or blower is operating, and the configuration of the exhaust system. The enclosures were typically designed with one engine or blower configuration in mind.
Another design factor which affected past designs was the incorporation of exhaust flow tubes into the enclosure. The flow tube had a flange mounted on one end which was then mounted to a traditional passive silencer or to the end of the existing exhaust pipe. The enclosure was built around the flow tube and therefore had to compensate for the volume taken up by the flow tube. This caused the enclosures to be large especially in the case of large flow tube diameters. The incorporation of the flow tube also caused the enclosure to be heated up by the radiation of heat from the tube. To reduce this heating effect, a heat shield was placed around the flow tube to serve as an insulator. This feature further caused the designs to increase in size.
Due to the different pipe sizes and the above constraints, the past exhaust silencing products were all very different. Using the same basic concepts in their designs, the prior designs all combine multiple speakers with specific acoustic parameters to achieve the desired amount of sound level required for noise cancellation on a designated exhaust pipe size. The 3-inch system typically uses two 8-inch speakers and the 10-inch and 22-inch systems typically use three 12-inch speakers to generate the anti-noise. The electronics required for the above systems were housed in a separate electrical box which is located away from the acoustic enclosure.
Though the current active noise cancellation systems of this type worked well, there were still several shortcomings. For example, the many different engine and blower pipe sizes and thermal differences required several different products to cover the desired market. Each product was completely different and all designs utilized only a few common parts. The existing systems which incorporate the flow tube, flange, and heat shield into the acoustic enclosure are very large. The size of the systems obviously increased with the size of the exhaust pipe. The past mufflers were made of welded sheet steel, the flow tubes were schedule 40 steel pipe, and the flanges were also steel. All of these factors caused the product to be very heavy.
Due to the large size and weight of the acoustic enclosures, the installation of the system was very difficult and required several individuals and/or heavy equipment. The cost of this equipment was the responsibility of the customer. Also, in most cases an additional flange had to be attached to the existing exhaust pipe to allow for mounting of the enclosure. Due to the large amount of extra weight placed at the ends of the exhaust pipes, extra braces often had to be installed to support the pipes or the enclosure.
Most of the exhaust silencing systems were designed for worst case (i.e. loudest exhaust noise applications) situations. Therefore, for most applications, the systems could generate much more anti-noise than was necessary for adequate noise attenuation. In some cases, the three speakers and three amplifiers along with a great deal of acoustic volume were often doing the same job that could have been accomplished with just two speakers, two amplifiers and less volume.
The past systems separated the electronics into separate boxes which were mounted at a remote location. The amplifiers, controllers, and power supplies were mounted inside these boxes. Since the enclosures were mounted apart from the electronics, the amount of cable needed was significant. There were up to three sets of speaker wire and one microphone cable. The microphone cable had to be shielded, due to the relatively low level of the microphone signal, to prevent electromagnetic corruption. It is also a common practice to keep the amount of speaker wire between the amplifier and the speaker to a minimum to prevent signal losses in long lengths of wire. The past systems violated this practice for most applications.
All of the reasons listed above contributed to the high cost of the past product. Due to the large sizes and high piece costs of the past product, it was difficult to order large quantities of any of the products. Uncertainty in the market made it difficult to know how many of each size to keep in stock. Therefore, most had to be built on demand which caused delays in delivery. The low volume builds caused the piece price to be high due to manufacturing scheduling issues and material purchases. The past products also required a great deal of welding and machining. These tasks required a great deal of very skilled labor which further added to the cost.
One method of aiding noise cancellation in the past was to use a device sometimes called a "mixing chamber." A simplified description of a mixing chamber is a box attached to the noise and anti-noise outlets. The box is closed around the two outlets and open to the atmosphere on the opposite end. The end of the box which is open to the atmosphere is generally larger than either the noise or the anti-noise outlets. The mixing chamber varies in noise/anti-noise configuration.
For the concentric configuration of past ANC products, the presence of the mixing chamber resulted in two effects. First, the chamber caused the anti-noise and exhaust noise sources to radiate more efficiently thereby increasing the output of each source. The mixing chamber was behaving similar to an acoustic horn by decreasing the impedance mismatch at the point where the source radiates into the atmosphere. Secondly, the chamber resulted in better cancellation of the noise source as compared to when the mixing chamber was not present.